I have used bulb for waterfalls for that blurred water effect at rather a late hour and for that I was very much just using it at random and seeing what effect different speeds would yeald and then reshooting after.
The other use of bulb I have seen is where you use it to shoot lightning or fireworks where you leave the shutter open (pointing at a very dark sky, not something for town work) and then wait for the flash of light - then quickly close the shutter. A cover is also handy to hold infront of the lens during gaps (mostly for firework shots as then you can decide to have the rocket in the shot or not-

You may be able to extrapolate from a wide aperture reading, if the light level is within your camera meter's sensitivity range. For example, if 2 s is the longest shutter speed the camera will do, and you get a reading of 2 s at f/1.4, you could calculate 32 s at f/5.6 in your head. If using film, you may then have to account for reciprocity departure (aka reciprocity failure).

Edit: I see that nynfortoo wrote the same thing as I was typing this.

If your camera meter is struggling to read because of low light leves, try taking a reading from a white card or other white object and open up two stops. This gives an effective two-stop increase in the sensitivity of your meter.